1. Field of the Invention
The present invention relates to oriented electrical steel sheet having a surface coating that includes a crystalline phase, and to a method of manufacturing same. The invention particularly relates to oriented electrical steel sheet in which core loss properties are markedly improved by a surface coating that has good adhesion and imparts a high degree of tension to the sheet base metal, and to a method for manufacturing same.
2. Description of the Prior Art
Oriented electrical steel sheet is extensively used as a material for magnetic cores. To reduce energy loss it is necessary to reduce core loss. JP-B-58-26405 discloses a method for reducing the core loss of oriented electrical steel sheet consisting of using a laser beam to impart localized stress to the sheet surface, following finish annealing, to thereby refine the size of the magnetic domains. JP-A-62-86175 discloses an example of a means of also refining magnetic domains so as not to lose the effect of stress relief annealing applied following core processing.
On the other hand, it is known that the application of tension to oriented electrical steel sheet degrades core loss properties. Oriented electrical steel sheet usually has a primary coating of forsterite formed during finish annealing (secondary recrystallization), and a secondary coating of phosphate formed on the primary layer. These layers impart tension to the steel sheet and contribute to reducing the core loss. However, because the tension imparted by the coating has not been enough to produce a sufficient reduction in core loss, there has been a need for coatings that will provide a further improvement in core loss properties by imparting a higher tension.
Methods of providing a greater improvement in core loss properties include the method described by JP-B-52-24499 which comprises following the completion of finish annealing by the application of the above primary coating and the removal of the oxide layer that is located near the surface of the steel sheet and impedes domain movement, flattening the base metal surface and providing a mirror surface finish which is then metal-plated, while the further provision of a tension coating is described by, for example, JP-B-56-4150, JP-A-61-201732, JP-B-63-54767, and JP-A-2-213483. While the greater the tension produced by the coating, the greater the improvement in core loss properties, the mirror surface finish produces a pronounced degradation in the adhesion of the coating to the steel sheet. This has led to the proposed use of various techniques to form the coating, such as physical vapor deposition, chemical vapor deposition, sputtering, ion plating, ion implantation, flame spraying and the like.
While it is recognized that films formed by physical vapor deposition, chemical vapor deposition, sputtering, ion plating and the like have good adhesion and that the tension thus imparted improves the core loss properties to a fair degree, these processes require a high vacuum and it takes a considerable time to obtain a film thick enough for practical application. Thus, such processes have the drawbacks of very low productivity and high cost, while for the purposes of forming coatings on electrical steel sheet, ion implantation and flame spraying cannot really be described as industrial techniques.
A coating method that is industrially applicable is the sol-gel method. JP-A-2-243770, for example, relates to the formation of an oxide coating, while JP-A-3-130376 describes a method of forming a thin gel coating on the surface of steel sheet that has been flattened, followed by the formation of an insulating layer. While it is possible to form coatings with such techniques, using the same application and baking processes as those of the prior art, as described in each of the specifications it is very difficult to form a sound coating having a thickness of not less than 0.5 .mu.m.
In order to obtain a coating of the thickness needed to impart a high degree of tension, repeated applications and heat treatments are required, and it has also been necessary to use another technique to form a coating on the sol-gel coating.